The present invention relates to a hydraulically operated power transmission including a hydraulic pump and a hydraulic motor which are interconnected by a closed hydraulic circuit, and more particularly to a hydraulically operated power transmission that can transmit driving power from a hydraulic pump to a hydraulic motor very quickly following a condition in which driving power was transmitted from the hydraulic motor to the hydraulic pump.
Various power transmissions are known in the art. One known design is a hydraulically operated power transmission which has a hydraulic pump driven by the driving power of an engine and a hydraulic motor rotated by oil pressure developed by the hydraulic pump. The rotative power of the hydraulic motor is transmitted to an input shaft of a device to which the driving power is to be transmitted.
In the known hydraulically operated power transmission, while the device is being driven by the driving power from the engine, oil pressure is transmitted from the hydraulic pump to the hydraulic motor, and the device is driven by the driving power generated by the hydraulic motor. If the oil pressure produced by the hydraulic pump is increased beyond a prescribed pressure level, the excessive oil pressure is released through a high-pressure relief valve.
When so-called engine braking is relied upon, the engine is driven by the driving power from drive wheels. At this time, since the hydraulic motor actuates the hydraulic pump, the oil pressure acting on the hydraulic motor from the hydraulic pump is released through a shuttle valve from a low-pressure relief valve into an oil tank. At the time of initiating acceleration from this engine braking condition, since the oil pressure is released via the shuttle valve from the low-pressure relief valve, it is necessary to switch over the shuttle valve to cut off the oil passage leading to the low-pressure relief valve, so that the oil pressure produced by the hydraulic pump can quickly be transmitted to the hydraulic motor.
If the shuttle valve is not quickly switched over, the oil pressure from the hydraulic pump would continuously be released, and hence would not immediately be applied to the hydraulic motor. Therefore, the hydraulic pump would be idly rotated, and the acceleration response would poor.